The invention relates to a method for positioning a shoe press/shoe calender in a paper machine.
The invention also relates to an arrangement for positioning a shoe of a shoe press/shoe calender in a paper machine, which arrangement comprises a shoe roll or equivalent which includes a shoe and hydraulic cylinders connected thereto for moving the shoe, and a support element to which the hydraulic cylinders and the shoe are attached.
As known in the state of the art, as loading rolls in a shoe press/shoe calender are used loading rolls in which a shoe is placed inside a polyurethane belt. The belt is attached at ends so as to be oilproof and the sock/belt is held tight around the shoe structure. The shoe structure comprises oil ducts and hydraulic cylinders for pressing the shoe against a backing roll/thermoroll. One known arrangement for a so-called belt calender is disclosed in FI patent application 943278, which discloses a device for polishing a paper web or a board web provided with a polishing zone through which the web is passed and which has a given thickness measured in the direction of the web to achieve a longer time of dwell of the web. The device is provided with two polishing surfaces defining the polishing zone on both sides, one of said surfaces being formed by a mantle of a machine-driven roll and the other of said surfaces being formed by a glide felt that runs around; provided with a glide shoe which is wrapped by the glide felt and which has a glide surface complementary to the mantle surface of the roll; provided with means for pressing the roll and the glide shoe against each other.
One problem in the known arrangements is that the position of the shoe is not known, wherefore, for example, when closing, the shoe may be positioned unevenly with the result that an uneven nip force is produced. This may lead, among other things, to breaking of the web.
One problem in calenders, at high speeds in particular, because of an undesirable position of the shoe, is an uneven/undesirable type of draw which is effective in the nip, in which connection uneven compression may break the web.
Moreover, a problem in presses under heavy load is that when the shoe is in an undesirable position, the backing roll and the belt will wear. In addition, problems may arise in the lubrication of the shoe because, when the shoe is out of line, a lubricating oil film is thinner on the edge which comes first into contact with the backing roll.
In a press section, problems may be caused by the fact that, when the nip is closed in an undesirable manner, the press felt may be crumpled.
In high-speed machines with a speed of over 1000 m/min, problems may arise in the controllability of felts and wires, if the shoe pulls felts/wires askew because of its incorrect position.
An object of the invention is to create a solution to the problems described above.
An object of the invention is to create an arrangement in which the position of the shoe is known and the shoe can be guided to a desired position, whereby the problems described above are eliminated.
With a view to achieving the objectives described above as well as those which will come out later, the method according to the invention is mainly characterized in that, in the method, the position of a shoe of a shoe calender/shoe press is measured and that, in the method, the position of the shoe is controlled based on the results of the measurement so as to be as desired in the direction of nip compression.
The arrangement according to the invention is in turn mainly characterized in that the arrangement comprises further at least two measuring devices for measuring the position of the shoe and means for guiding the shoe as desired in the direction of nip compression based on the results obtained by means of the measuring devices. In accordance with the invention, the means for controlling the shoe based on the results obtained by the measuring devices include a processing unit, which can be, for example, a programmable logic or a process or computing station (CPU) of a distributed automation system. The signals computed by the processing unit can be passed to valves which control the movement of cylinders controlling the movement of the shoe, for example, by using standard current or voltage signals of instrumentation or some automation field bus.
In accordance with the invention, the position of the shoe in a shoe calender/shoe press is measured by at least two measuring devices suitable for position measurement, for example, a linear sensor. The position measuring sensors are placed most advantageously close to the edges of the driving and tending sides and in wide machines also in the middle area. In that connection, the position of the shoe is found out, and the nip is caused to close in a desired position since the movements of the hydraulic cylinders can be regulated accurately based on the measurement results obtained.
In accordance with the invention, the shoe of the shoe press/shoe calender is provided with at least two position measuring sensors and the movement of the shoe is regulated based on data provided by the sensors, for example, utilizing a computing algorithm, and the hydraulic cylinders are controlled to operate such that a desired movement and position of the shoe is assured in the direction of nip compression.
When the nip is closed/opened in accordance with the invention, there is no risk of the web breaking, because the position of the shoe is known precisely when the measurement result provided by the position measuring sensors has been received, and the shoe can be positioned in a desired position. In accordance with the invention, also after a web break or downtime, it is possible to control the oil flows of the hydraulic cylinder based on the position data obtained from the position measuring sensors such that the nip is controlled to close in a desired manner, whereby the position of the shoe is caused to be as desired.
In the arrangement according to the invention, the shoe can thus be controlled to close in an optimal manner with respect to the running situation.